Old

• var pi = 3.14159;

New

1. const pi = 3.14159;
2. let radius = 5;

Const

Short for constant – constant variables.
const is used for values you never want reassigning.

const pi = 3.14159;
pi = 3.14 // Will throw a TypeError

You can change and add properties to an object that’s stored as a constant. But, you can’t assign a new value, even a new object to that variable. For example, you can modify an array that’s assigned as a constant. You just can’t assign a NEW value (like a new array) to that variable.

Passes successfully

const student = { 
  firstName: "Rhonda",
  lastName: "Salamon"
}
student.lastName = "Gertz";

Throws an error

const animal = { 
  type: "Dog",
  name: "Rover"
}
animal = { 
  type: "Cat",
  name: "Grumpy"
}

Let

Using the var keyword in for loops can cause unexpected behavior. var usage should be avoided from now on. let works like var, allowing you to re-assign variables, but unlike var it has block level scoping.

for ( let i = 0; i < 10; i++ ) {
  console.log(i);
}

Const & Let

const should be your first choice when creating a variable.
While you can use any of these to create a variable, using const as your first choice will help you prevent errors like overwriting the value in a variable that shouldn’t change.

function secondsWorked(hours) {
  let totalTime;
  const minutesInHour = 60;
  const minutesInSecond = 60;
  totalTime =  hours * minutesInHour * minutesInSecond; 
}
console.log( secondsWorked(40) );